1. Field of the Invention
This invention relates to methods of fracture acidizing producing zones in subterranean formations.
2. Description of the Prior Art.
A variety of methods have been developed and used successfully for stimulating the production of oil and gas from subterranean formations. One commonly used stimulation method is known as fracture acidizing. Fracture acidizing is generally utilized for stimulating producing zones in subterranean formations formed of fracturable and acid soluble calcareous rock having relatively low permeability. Because of the low permeability, hydrocarbons contained in the zone normally flow into the well bore penetrating the zone at a low rate.
A fracture acidizing procedure generally comprises hydraulically fracturing the formation rock in a subterranean producing zone at above fracturing pressure using an acid which dissolves the fracture faces in such an uneven manner that when the fracture is closed and the zone is produced, flow channels are provided through which hydrocarbons contained in the zone more readily flow to the well bore. A problem often encountered in such fracture acidizing procedures involves over-etching or softening the entirety of the fracture faces. That is, the acid readily dissolves portions of the rock faces of the fractures, but when the pressure on the formation is reduced and the fractures are caused to close, the dissolved portions are often restricted as a result of the fracture faces being over-etched or softened and crushed against each other. Thus, reduced hydrocarbon flow capacity can result due to the entirety of the rock faces being exposed to the acid and becoming softened.
More recently, an improved method of fracture acidizing a subterranean producing zone has been developed and used which is described in U.S. Pat. No. 3,768,564 issued on Oct. 30, 1973 to Fredrickson and Knox. In accordance with that method, one or more fractures are created in a subterranean producing zone, the fractures are allowed to close prior to prolonged contact with acid and then additional acid is flowed between the closed faces of the fractures whereby high capacity flow channels are produced. The faces of the fractures are prevented from being over-etched and softened by the acid whereby portions of the fracture faces are left intact for providing support when the fractures are closed. More specifically, after fractures are created and extended in a subterranean producing zone, the faces of the fractures are contacted with acid while maintaining the fractures in an open position. Prior to over-etching the fracture faces, the fractures are allowed to close and acid is flowed through flow channels formed therein while the fractures are closed to enlarge the channels and increase their flow capacity.
While the method described in U.S. Pat. No. 3,768,564 has been utilized successfully over the years, it has been difficult to produce fractures containing flow channels which extend relatively long distances from the well bore into the subterranean producing zone. This is because fractures may tend to grow upwardly, downwardly, and outwardly directions adjacent to the well bore where the hydraulic pressure exerted on the formation is the greatest rather than in outwardly directions only away from the well bore as is desirable.
Thus, there is a need for an improved method of fracture acidizing a producing zone in a subterranean formation whereby fractures containing flow channels can be produced in the zone which extend greater distances outwardly from the well bore.